62 | Astronomy Now | June 2012

The 1761 transit of Venus saw the world’s first major multinational science experiment end in disaster, writes Andrea Wulf, author of Chasing Venus, a new book that tells the story of the quest to measure the transit and in turn the scale of the Solar System. What lessons did the scientific community learn from these misadventures for the second transit in 1769?

On 5 June 1760, like every Thursday afternoon, the fellows of the Royal Society

made their way to Crane Court, a little cul-de-sac off Fleet Street in London to assemble for their weekly meeting. As they passed the glittering world of shopping on the Strand and Fleet Street, they saw precious wares on display, a spectacle of objects that testified Britain’s reach across the globe and her manufacturing prowess – from silver teapots, new telescopes and heaps of delicate lace to pyramids of pineapples and grapes.

When the fellows arrived at their headquarters, they exchanged the latest news as they climbed up the stairs to their meeting room. As always, it took a moment for all fellows to settle and for the chatter to quieten down. Then, after the attendance was noted, one of them stood up and read a letter that would set in motion a chain of events that would occupy the Royal Society and scientists across the world for more than a decade.

The Royal Society was the most important scientific forum

in Britain. Since its foundation in the 1660s ‘for the improvement of natural knowledge by Experiment,’ it had become the nexus of enquiry and Enlightenment thinking. The letter that was read on 5 June was the beginning of what would be the

first global scientific collaboration, bringing together hundreds of men who would look at the stars to understand the dimensions of the Solar System. Joseph-Nicolas Delisle, the official astronomer to the French Navy and a member of the French Académie des Sciences in Paris had written, asking the British to remember what the late Astronomer Royal Edmond Halley had said 44 years previously: they were to observe the transit of Venus that would occur exactly one year later, on 6 June 1761.

Transits of Venus are some of the rarest predictable astronomical events. They always occur in pairs – eight years apart – but then it takes more than a century before they are seen again. For a few hours the brightest planet in the night sky would appear as a perfectly black circle marching across the burning face of the Sun. Halley believed that measuring the exact time and duration of this rare celestial encounter would provide the data that astronomers needed to calculate the distance between the Earth and the Sun – the key for

The Royal Society and the 1761 transit of Venus

▼ How the 1761 and 1769 transits of Venus appeared on the Sun. AN graphic by Greg Smye–Rumsby.

Focus: the transit of Venus

the Royal Society waS Sending theiR aStRonomeRS into waR zoneS. nothing,

it Seemed, waS to Stand in the way oF the theiR PlanS to RecoRd the tRanSit oF

VenUS.

▲One of Jean-Baptiste Chappe d'Auteroche's enclosed sledges in which he travelled from St Petersburg to Siberia for the 1761 transit. Image: Wellcome Library, London

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The Royal Society and the 1761 transit of Venus

▲ James Ferguson’s maps showing where and when differing locations on the Earth could see the 6 June 1761 transit of Venus. Image: Wellcome Library, London.

▲ A drawing of the transit of Venus as predicted for London, India, St Helena and Bencoolen in 1761, published in James Ferguson’s 1770 book, Astronomy Explained Upon Sir Isaac Newton’s Principles. Image: Wellcome Library, London.

the size of the Solar System and the holy grail of astronomy.

If several people around the world were to view the transit from different places as far apart as possible, they would observe Venus traversing the Sun along a slightly different track – dependent upon those observers’ locations in the Northern and Southern Hemispheres. The further south the location was for the transit in 1761, the longer the duration of the transit appeared and the further north the shorter. With the help of trigonometry, these different tracks (and the differences in the duration of Venus’s march across the Sun) could then be used to calculate the distance between the Sun and Earth. No nation alone would be able to compile enough data. For the first time scientists from across the globe would have to work together, combining their results, with the northern viewings being the counterpart to the southern observations.

Four corners of the worldThe Royal Society was quick to follow Delisle’s instructions and within five weeks they had decided to dispatch two expeditions to far–flung corners of the British Empire: to St Helena in the South Atlantic (the most southerly territory under British control) and to Bengkulu, a East India Company trading port on Sumatra. By the time funding, transport and instruments were secured, there was not much time left. On 6 January 1761 Charles Mason and Jeremiah Dixon (later famously known for the eponymous Mason–Dixon Line) finally sailed, only to be attacked by the French just off the coast of Plymouth, leaving their vessel destroyed, eleven dead and forty–two wounded. With the Seven–Years’ War tearing apart Europe and the colonial possessions, the Royal Society was sending their astronomers into war zones.

Mason and Dixon were so terrified that they pleaded with the Royal Society to be dismissed from their mission – but to no avail. The fellows were appalled by their cowardice and ordered the two disobedient astronomers to “enter upon the Voyage”. To make clear that this was not just a polite nudge, they threatened that Mason and Dixon would be prosecuted in court. The two astronomers were going to Bengkulu, if they wanted to or not

– otherwise they would be punished as mutineers “with the utmost Severity of Law”. Nothing, it seemed, was to stand in the way of the Royal Society’s plans.

Meanwhile, Nevil Maskelyne (who would later become Astronomer Royal) was on his way to St Helena, loaded with the best instruments available but also with more than 100 gallons of wine and spirits. The British, however, were not the only nation sending expeditions to remote places. The French dispatched astronomers to Pondicherry, Rodrigues (an island in the Indian Ocean) and Siberia while the Swedes had organised viewings in the far north in Lapland. Never before had so many nations and scientific societies

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Focus: the transit of Venus

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Dixon had ignored their orders and watched the transit at the Cape of Good Hope because the French had taken Bengkulu – the Royal Society remained blissfully ignorant of the fact that their expedition had ended prematurely. Maskelyne had arrived safely in St Helena but only saw an overcast sky. In Bavaria one observer cried that the Sun was ‘devoured by clouds’; in Russia an astronomer struggled with strong winds that shook his telescope, while others stared into a curtain of rain. Everybody hoped to catch a glimpse of ‘Madame Venus’, but even those who looked into a clear sky despaired at what they saw. When the reports were collected at the scientific societies in Europe, it became obvious that the observations had not been as successful as they had hoped. Comments like “doubtful”, “not sure” and “not certain” peppered the letters. The problem was that Venus had not moved swiftly onto the Sun but had lingered for up to a minute, seemingly glued to the edge, unwilling to embark on her precious path – the so–called black–drop effect that made it impossible to determine the exact beginning and end of the transit that was the essential data they needed. The edge of Venus was ‘trembling’, some

attempted to work together on one single endeavour. British instrument-makers worked around the clock to dispatch the necessary instruments to astronomers and learned academies across Europe. Delisle published a map of the world that depicted the best viewing locations – copies of which he dispatched to colleagues in Holland, Italy, Switzerland, Germany, Austria, Sweden, Constantinople, Russia and Britain.

Bad luckThis army of astronomers made for strange explorers. Many of them were middle–aged and had lived a life that was more ruled by the repetitive labour of nightly observations than by swashbuckling adventures. They might have not looked like heroic globetrotters, but as they chased Venus they did with extraordinary intrepidity and bravery. The French astronomer Chappe d’Auteroche traveled 4,000 miles from Paris to Siberia, much of it struggling through ice and snow. Uncountable times his carriages and sledges – loaded with more than half a ton of instruments – crashed through frozen rivers or got stuck in snow drifts. He arrived just in time, only to be threatened by Siberian peasants who wanted to murder him because they believed that his large telescopes had caused the severe spring floods that were destroying their houses. His colleague Le Gentil, who was traveling to the French trading port Pondicherry in India, was even more unfortunate because after a journey that took more than a year and that was beset with enemy attacks, tropical storms and life–threatening diseases, he discovered that the British had besieged the town. He was stuck on a rolling boat in the middle of the Indian Ocean where his pendulum clock (which he needed to time the transit) was of no use at all, making any observation obsolete.

As the day of the transit arrived, more than 250 official observers pointed their telescopes to the sky, hoping that no clouds would veil the Sun. Mason and

▲ Captain Cook’s ‘Fort Venus’ in Tahiti, by Sydney Parksinon in A Journal of a Voyage to the South Seas, in His Majesty’s ship, the Endeavour, 1784. The observatory tent is the round structure in the middle with the flag. Image: Wellcome Library.

▼ The Royal Society on Crane Street, London, circa 1878. Image: Wellcome Library, London.

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reported, while others described a halo–like ring around the planet. Even astronomers who watched side–by–side recorded the times with a twenty–two second difference. Far too much for a precise science such as astronomy.

Nothing about the transit seemed straightforward and the measurements were fraught with mistakes. The astronomers went back and forth, but no matter how much they calculated and played with the numbers (and some did fiddle the figures in an attempt to make it all fit), their results varied widely – making a whopping 32 million kilometres (20 million miles) difference. But all was not lost because there was the second transit on 3 June 1769 – it was their last chance. They had eight years to prepare.

Second chanceOnce again astronomers from across the world worked together. The Royal Society took the leading role and the British alone organised eighty observers at thirty viewing stations in Britain and sixteen abroad (not counting the North American colonies). Captain James Cook sailed to Tahiti on the Endeavour to observe the transit there, others observed at the Arctic Circle and Hudson Bay. The French dispatched Chappe d’Auteroche once again, but he arrived so late in Baja California that he set up his telescopes at the first possible location: a typhus–ridden mission. It was a decision that he paid for with his life when he died of the disease shortly after the transit. The doomed Le Gentil also failed again to see Venus but others were luckier. As the results poured in, the astronomers were able to determine the distance between Earth and Sun within a range from 149.5 million kilometres (92,900,000 miles) to 155.9 million kilometres (96,900,000 miles) – very close to today’s value of 149.6 million kilometres (92,960,000 miles).

Maybe most importantly, the achievements of the transit projects changed the world of science. For the first time an international community of scientists banded together on a global project despite wars and their patriotic agendas. Their interests had transcended national boundaries and for the first time governments had funded large–scale scientific projects. Today we believe that global projects are a modern day phenomenon but the roots of such collaborations lie with the transit astronomers. So, when you see Venus marching across the face of the Sun on 6 June 2012, spare a thought for these intrepid astronomers.

Andrea Wulf’s Chasing Venus: The Race to Measure the Heavens is published in eight countries in conjunction with the transit in June 2012. Read our review on page 76!

Thorbern Bergman’s observations of the 1761 transit of Venus as seen from Upsal in Sweden, appearing in Philosophical Transactions of the Royal Society volume 52. Figures one and two show the luminous ring around Venus as the planet moved onto the Sun’s disc. Figures five and six show the black drop effect. Image: Wellcome Library, London.

A large observatory telescope being used by an astronomer of the time. Such instruments will have been used to record the transit of Venus during the 1760s. This engraving is by the eighteenth century artist James Basire. Image: Wellcome Library, London.

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